An ongoing and growing problem associated with today's mechanized society is the mushrooming noise pollution that exists in all walks of life. The sound of highway traffic may spill over from the highway into adjoining neighborhoods, creating a constant irritation to the people who live there. Big city traffic sounds, sirens, and honking all contribute to the stress of urban life.
Jet sounds close to airports can actually cause hearing loss to those in close proximity to the jet engines. Concrete blast barriers are typically constructed around engine run-up areas to help protect the hearing of individuals who work or live closeby.
Night clubs which feature live entertainment or loud “canned” music draw many complaints from property owners or nearby residents who suffer from the night club noise late at night, contributing to the health problem of sleep deprivation which already affects many individuals. Some night club owners attempt to solve the problem by installing cheap, Styrofoam insulation, which may be highly flammable. At least one recent night club fire was tragically exacerbated by the rapid combustion of such flammable sound insulation. Thus, it would be desirable to provide acoustic panels which are not only effective, but which are safe.
Existing Designs
In the “high-tech” arena, significant advances have been made in the area of noise attenuation. It has been discovered that not only can unwanted sound be physically blocked by barriers, but that such barriers can actually be designed to absorb sound. Such barriers are typically engineered specifically for the application at hand. For example, sound absorptive devices have been designed and installed in aircraft auxiliary power unit inlets and exhausts, in the International Space Station, in military vehicle exhausts, and in other noise control products in aerospace, military, and industrial applications.
Such products typically require analysis of the specific application, acoustic and mechanical design of the sound absorptive product itself, and finally custom-fabrication of the item. One such product is illustrated in FIGS. 1 and 2. FIG. 1 is a front quarter isometric view of acoustic silencer 2. FIG. 2 is a cross-sectional view of acoustic silencer 2 taken at section II—II of FIG. 1.
As may be observed in FIGS. 1 and 2, acoustic silencer 2 comprises acoustic silencer duct 4 through which intake or exhaust gasses (for an auxiliary power unit, for example) move. Sound associated with the auxiliary power unit exists inside acoustic silencer duct 4 and is absorbed into acoustic silencer 2 as indicated by arrows 14.
Acoustic silencer 2 comprises screen 6, felt layer 8, honeycomb layer 10 and back plate 12. Honeycomb layer 10 serves to offset felt layer 8 and screen 6 away from back plate 12 by a standoff distance 11. It has been determined that an acoustic silencer 2 will absorb sound having a wavelength equal to four times standoff distance 11. For example, 6800 hertz sound has a wavelength of 2 inches. Therefore, the optimum standoff distance 11 to absorb this sound is ½ inch, because 4 times ½inch=2 inches. Thus, where 6800 hertz sound is to be absorbed, standoff distance 11 would be ½ inch.
The acoustic silencer 2 illustrated in FIG. 2 used a metal screen 6 having approx. 60 wires/inch. Felt layer 8 was a layer of felt. Honeycomb layer 10 was a fiberglass phonetic resin or aluminum, and comprised hexagonal cross-section through passages. Back plate 12 was fiberglass-phenolic, metal, fiberglass-epoxy or carbon fiber-epoxy.
While the acoustic silencer 2 depicted in FIGS. 1 and 2 provided good sound absorption qualities, it was expensive and had to be designed specifically for each application. Accordingly, it would be desirable to provide a sound absorbing panel which could be pre-manufactured to standard sizes, and ready for use in a variety of applications.